Data is communicated using code division multiple access systems by combining the data with a user specific spreading code and conveying the combination using radio signals which are transmitted between a transmitter and a receiver. At the receiver, the data is recovered from the radio signals by comparing the received radio signals with the user specific spreading code which is known to the receiver. In this way, a plurality of transmitters associated with a corresponding plurality of users may be arranged to contemporaneously communicate data from the transmitters to receivers which detect the data by comparing the radio signals with the unique user specific spreading code associated with each of the users.
A result of combining the data with a user specific spreading code, is that a bandwidth of the resulting radio signals is greater than that which would be the case if the data were carried alone by the radio signals. As a result, the frequency bandwidth occupied by code division multiple access signals is greater than a coherence bandwidth of the radio communications channel through which the signals pass. As a result code division multiple access signals are provided with an advantageous effect in that the signals themselves exhibit frequency diversity in that parts of the signals are communicated in frequencies which suffer fading independently.
Code division multiple access is used in both second and is now being proposed for third generation mobile radio telephone systems. A characteristic of a frequency bandwidth in which radio signals are transmitted in second and third generation mobile radio telephone systems, is that the radio signals arrive at receivers of the system via multiple paths. As a result, the received signals exhibit inter symbol interference which must be cancelled in order for the data to be recovered. A further advantage of code division multiple access, is that the increased bandwidth of the radio signals allows for each of the paths via which the radio signals reach the receiver to be individually determined and the radio signal energy for each of the paths to be recovered and combined to facilitate detection of the communicated data. A receiver which operates to effect the detection of individual paths via which radio signals reach the receiver is known to those skilled in the art as the rake receiver.
One known rake receiver which operates to combine conventional rake reception techniques with multi-user detection is described in a published article entitled ‘A multi-user receiver for code division multiple access communications over multipath channels’, by U. Fawer and B. Aazhang published in IEEE Transactions on Communications, Vol. 43, No. 2/3/4, February/March/April 1995 at pages 1556–1565. This known receiver is provided with a plurality of data detectors known as rake fingers which together form a rake receiver. Each of the fingers of the rake receiver are assigned to one of the plurality of users and one of a plurality of temporal displacements corresponding to one of a plurality of paths via which the radio signals reach the receiver. Furthermore the disclosed receiver is provided with a multi-user data detector which is fed with symbols detected by each of the rake fingers and serves to effect an iterative process from which data from each of the individual users is recovered by treating the detected symbols as either wanted data which is therefore recovered by the iterative process or interfering symbols which are arranged to be cancelled from the wanted data.
Rake receivers for code division multiple access radio signals suffer from multiple access interference, which is generated from other unwanted radio signals and may be described for the detection of each symbol individually by a cross-correlation between radio signals for different users during one symbol period before a symbol estimate is generated by each of the rake fingers. Furthermore interference is also caused in each of the rake fingers from radio signals reaching the receiver via other paths from the same user which have different temporal displacements which do not correspond with the temporal positions at which each rake finger is recovering the data from the radio signals. As such, self inter symbol interference is caused which may be described for the detection of each symbol individually by the auto-correlation of the radio signals for a specific user with a time shift equal to the difference in the temporal displacement between that operated by the rake finger and other interfering paths during one symbol period before the rake finger recovers an estimate of the symbol.
In EP 0491668A and WO 95/22209 CDMA demodulators are described which decode the received composite signal in the order of strongest to weakest signal strength. The number of decoder elements or data detectors is the same for every user spreading code.